Many months into the COVID-19 pandemic, the coronavirus is still spreading uncontrolled through the U.S. Public health authorities including the U.S. Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO) tell us to remain six feet apart, wash our hands, disinfect frequently touched surfaces, and wear masks. But compliance with these measures—especially masks—is mixed, and daily we hear of cases where people do not know how they were infected. We hear about superspreading events, where one person infects many, happening in crowded bars and family gatherings, but not at outdoor demonstrations. Beaches in cities like Chicago are closed, but gyms and indoor dining at restaurants have reopened. It is no wonder the public is confused.
It is critical to have a clear physical description of the ways in which COVID-19 is transmitted, so that individuals and institutions are able to visualize it and will understand how to protect themselves. Contrary to public health messaging, I, together with many other scientists, believe that a substantial share of COVID-19 cases are the result of transmission through aerosols. The evidence in favor of aerosols is stronger than that for any other pathway, and officials need to be more aggressive in expressing this reality if we want to get the pandemic under control.
There are three possible ways the virus is transmitted, of which two have been emphasized by the WHO and the CDC. The first is through “fomites,” objects that are contaminated with the virus (which could include someone else’s skin). Early in the pandemic, concern over fomite transmission drove some people to bleach groceries and packages. The CDC now says fomites are a possible means of transmission, but likely not one that is major. For example, an intensive handwashing program in the UK led to only a 16% reduction in transmission. Significantly, other viruses that, like SARS-CoV-2 (the one that causes COVID-19), have a lipid envelope, do not survive long on human hands. That means someone would need to touch their eyes, nostrils, or mouth a short time after touching a contaminated surface in order to contract the novel coronavirus.
The second possibility for how COVID-19 spreads is through droplets, small bits of saliva or respiratory fluid that infected individuals expel when they cough, sneeze, or talk. Droplets—which the WHO and CDC maintain is the primary means of transmission of COVID-19—are propelled through the air, but fall to the ground after traveling 3-6 feet. However, published research, which has been replicated, shows that droplets are only important when coughing and sneezing. But when it comes to talking in close proximity, which appears to play a major role in COVID-19 transmission, droplets are less important than the third potential pathway: aerosols. Many diseases, including COVID-19, infect most effectively at close proximity. Since droplets are visible and fall to the ground between 3-6 feet, we can readily see and understand this route of infection. In fact, it was thought for decades that tuberculosis was transmitted by droplets and fomites, based on ease of infection at close proximity, but research eventually proved that tuberculosis can only be transmitted through aerosols. I believe that we have been making a similar mistake for COVID-19.
“Aerosol” (sometimes referred to as “airborne”) transmission is similar to droplet transmission, except that the bits of fluid are so small that they can linger in the air for minutes to hours. To understand the scale of aerosols, the diameter of a human hair is about 80 microns, and aerosols smaller than about 50 microns can float in the air long enough to be inhaled. SARS-CoV-2 is only 0.1 microns in diameter, so there is room for plenty of viruses in aerosols.
Fomites and droplets have dominated our everyday understanding of COVID-19 transmission. While the WHO and CDC both state that aerosols could lead to transmission under highly specific situations, both organizations maintain that they are less important. I believe this is a significant mistake and on July 6 I, along with 239 scientists, appealed to the WHO to reevaluate their stance. WHO updated its position in response, but the agency’s language continues to express skepticism of the importance of this pathway.
The unwillingness to acknowledge the likelihood that aerosols are a major means of COVID-19 transmission can be traced to the legacy of Dr. Charles Chapin, an American public health researcher. Trying to bury once and for all the theory of miasmas, ghostly clouds of disease, he argued in his seminal 1910 book The Sources and Modes of Infection that aerosol transmission was nearly impossible. “It will be a great relief to most persons to be freed from the specter of infected air, a specter that has pursued the race since the time of Hippocrates,” Chapin wrote. The impact of his book was fortuitous in a way: it came at a time when enough evidence about the transmission of different infectious diseases had accumulated since the discovery of germs by Pasteur in the 1860s, but before we had the technology to measure aerosols. Chapin’s notions became the paradigm of infectious disease transmission, which has dominated until now.
Given this deeply held disbelief of aerosol transmission, just a few diseases, including measles and chickenpox, have been accepted as being transmitted through aerosols—and only because these are so transmissible that the evidence could not be ignored by the medical community. Some less-contagious respiratory diseases, like influenza, were described as due to droplet and fomite transmission, even when they clearly had an aerosol component. That stance has, over the years, created an unfounded perception in health care that any disease that is transmitted through aerosols has to be extremely contagious. But 110 years later, the nuances and importance of aerosol transmission of respiratory diseases are finally becoming mainstream.